1. Field of the Invention
The present invention is generally concerned with an uplink frame at the transcoder rate adaptation unit (TRAU)/base transceiver station (BTS) interface in a cellular mobile radio network. In this type of network the uplink frame is transmitted by a BTS to a TRAU, and the invention also concerns a BTS for producing a frame of this kind.
2. Description of the Prior Art
The limited frequency bandwidth available in mobile radio networks necessitates the processing, or coding, of speech signals to reduce the bit rate. In a fixed network such as the public switched telephone network (PSTN) the bit rate on a PCM multiplex channel is typically 64 kbit/s in the case of a call that has been set up. To reduce this bit rate the GSM (Global System for Mobile communications) network uses speech signal processing by an RPE-LPT (Regular Pulse Excitation, Long Term Prediction) coder to yield a processed segment of 260 samples representing 20 ms of speech and defining a bit rate of 13 kbit/s. A speech channel received from the PSTN at 64 kbit/s is therefore processed by the GSM network to produce a speech channel at 13 kbit/s and, conversely, a speech signal at 13 kbit/s received from a mobile station is processed in the GSM mobile network to provide a 64 kbit/s channel into the PSTN. In a GSM network of this kind the functional entity which converts 64 kbit/s speech channels into 13 kbit/s channels and vice versa is the TRAU.
FIG. 1 is a diagrammatic representation of a transmission system using the architecture of the GSM mobile network. It includes in a cascade arrangement a mobile services switching center (MSC), a TRAU 1, a base station controller (BSC) 2, two BTS 30 and 31 and mobile stations (MS) 4. The TRAU 1 is connected to the BSC 2 by a link 2a and the BSC 2 is connected to the two BTS 30 and 31 by respective links 30a and 31a. As already explained, the TRAU 1 processes data (e.g. speech) channels at 64 kbit/s to produce 13 kbit/s channels and vice versa. It receives from and transmits to the PSTN, via the MSC, 64 kbit/s multiplexed data channels carried by a transmission link 1a. It transmits to and receives from the BTS 30 and 31, via the BSC 2, 16 kbit/s frames TR.
In a TR frame the wanted data, such as the speech signal for a call setup to a mobile station 4, defines a bit rate of 13 kbit/s within the overall bit rate of 16 kbit/s of the frame, the remaining 3 kbit/s being used to transmit synchronization and control signals between the BTS 30, 31 and the TRAU 1.
In the uplink direction from the mobile station 4 to the PSTN these uplink frames TR are produced by a BTS 30 or 31 from 13 kbit/s speech data received from a mobile station. The BTS 30 or 31 inserts the synchronization and control signals accounting for 3 kbit/s to form each 16 kbit/s uplink frame TR from the speech sector accounting for 13 kbit/s. The uplink frames TR are received by the TRAU 1 which subtracts the synchronization and control signals inserted by the BTS 30, 31 and converts the resulting 13 kbit/s speech signal into a 64 kbit/s signal transmitted to the PSTN.
In the downlink direction from the PSTN to a mobile station 4 the TRAU 1 inserts synchronization and control signals to form downlink frames at 16 kbit/s from the stream of wanted data at 13 kbit/s. This stream results from processing by the TRAU 1 of a 64 kbit/s speech signal received from the PSTN. The downlink frames TR are received by the BTS 30, 31 concerned which subtracts the synchronization and control signals inserted by the TRAU 1 and transmits in a communication channel set up to the mobile station 4 via an antenna 30b, 31b only the speech data signals.
As shown in FIG. 2, each uplink frame TR transmitted from a BTS 30 or 31 to the TRAU 1 includes 8×40=320 bits defining 20 ms of speech. As specified in ETSI Recommendation (European Digital Cellular Telecommunications System/Inband Control of Remote Transcoders and Rate Adapters, October 1993), the uplink frame includes, firstly, a wanted information field in the form of a first control field CC1, a plurality of data fields CD and a second control field CC2, and, secondly, a frame synchronization pattern MST and 19 periodic synchronization patterns MSP1 through MSP19 between two successive data fields.
The top line and the lefthand column in the FIG. 2 table give, for a given bit in the frame, the number (1 through 40) of the byte including that bit and the number of said bit within that byte.
The frame synchronization pattern MST occupies the first two bytes (bytes 1 and 2) of the frame TR and comprises 2×8=16 bits at “0”, the “0” state being called the first state. The periodic synchronization patterns MSP1 through MSP19 each comprise a bit at “1”, the “1” state being called the second state, and each occupies a respective bit of an odd-numbered byte in the frame, starting at byte 3. Accordingly, the synchronization pattern MSP1 occupies the first bit of byte 3, the synchronization pattern MSP2 occupies the first bit of byte 5, and so on recurrently up to synchronization pattern MSP19 which occupies the first bit of byte 39. The control field CC1 comprises 15 bits C1 through C15 and occupies the third and fourth bytes of the frame TR, except for the first bit of the third byte (byte 3), carrying the pattern MSP1.
The control field CC2 comprises six bits C16 through C21 and occupies the last two bits of byte 39 and first four bits of byte 40. The bits in the control fields CC1 and CC2 define the frame type (speech) and the channel type (bit rate, etc) and also carry time alignment information, etc.
In the context of the invention, the emphasis is on the control bit C12 carried in a subfield of control field CC1 and shown shaded within field CC1. In the prior art, this bit represents a bad frame indication representative of the validity of the data D transmitted in the data fields CD.
In an uplink frame TR, a BTS 30 or 31 sets this bit C12 to “1” or “0” to tell the TRAU 1 that said frame TR carrying this bit C12 is a bad frame or a good frame, respectively. The TRAU 1 includes means for applying specific processing if the uplink frame TR is indicated as a bad frame, which avoids the generation of 20 ms of invalid speech signal that would otherwise be transmitted at 64 kbit/s to the PSTN. The bit C12 is inserted into the uplink frame TR by a channel codec unit (CCU) in the BTS 30 or 31 that produced the frame.
Referring again to FIG. 1, at the time of a change of cell (internal handover) in response to a mobile station (MS) 4 crossing the imaginary boundary B between two cells with which the respective BTS 30 and 31 are associated, it can be seen that the location of the prior art bad frame indication bit C12 is less than optimal and leads to the risk of incorrect interpretation of data bits by the TRAU 1. To explain this, it is assumed (for example) that prior to the handover the mobile 4 is connected to the BTS 30 which produces the uplink frames TR to the TRAU 1 and that after handover the mobile station 4 is connected to the BTS 31 which produces the uplink frames relating to the call set up with the mobile station transmitted to the TRAU 1. The “source” BTS 30 and the “target” BTS 31 are not synchronized in terms of frequency or time in connection with the frames transmitted. During handover, the TRAU 1 therefore receives a first frame TR portion transmitted by the source BTS 30 and a second frame TR portion transmitted by the target BTS 31. If handover occurs after the synchronization pattern MST and before bit C12, relative to the first frame transmitted by the source BTS 30, there is a high probability that a bit of the second frame transmitted by the target BTS 31 and interpreted by the TRAU 1 as bit C12 of the first frame will assume a “0” state indicating to the TRAU 1 that the frame received is good, whereas it is in fact bad. On the other hand, the bit C12 transmitted by the source BTS 30 and indicating a bad frame is not received by the TRAU 1.
Thus, in the prior art, the uplink frame structure defined at the TRAU/BTS interface is less than optimal in connection with internal handover between two BTS using the same TRAU. With this frame structure, a frame can be indicated to the TRAU as “good” during handover whereas it is in fact “bad”.
The invention is directed to overcoming the aforementioned drawback by providing an uplink frame at the BTS/TRAU interface that makes such incorrect interpretation of the bad frame information by the TRAU virtually impossible.